The Weixi High-silica Granitoids in the Central Sanjiang Orogenic Belt,Southwest China:Implications for Growth of the Continental Crust

High-silica granitoids record the formation and evolution of the continental crust.A new intrusive complex has been recognized among silicic volcanic rocks of the Weixi arc,Southwest China.The intrusions consist of granites,granitic porphyries,and granodiorites.Zircon U-Pb age data indicate that the Weixi granitoids formed at 248-240 Ma and were coeval with silicic volcanic rocks of the Weixi arc.The Weixi granitoids are enriched in Rb,Th,and U,depleted in Ba,Sr,Nb,Ta,and Ti,and have high light/heavy rare earth element ratios and slightly negative Eu anomalies.The Weixi granitoids have negative ε_(Nd)(t)values(-9.8 to-7.8)and negative zircon ε_(Hf)(t)values(-12.02 to-5.11).The geochemical and isotopic features suggest the Weixi granitoids were derived by partial melting of ancient crustal material.The Weixi granitoids and silicic volcanic rocks were derived from the same magma by crystal accumulation and melt extraction,respectively,and they record the formation of a continental arc in the central Sanjiang orogenic belt.

Iteration and evaluation of shale oil development technology for continental rift lake basins

By benchmarking with the iteration of drilling technology,fracturing technology and well placement mode for shale oil and gas development in the United States and considering the geological characteristics and development difficulties of shale oil in the Jiyang continental rift lake basin,East China,the development technology system suitable for the geological characteristics of shale oil in continental rift lake basins has been primarily formed through innovation and iteration of the development,drilling and fracturing technologies.The technology system supports the rapid growth of shale oil production and reduces the development investment cost.By comparing it with the shale oil development technology in the United States,the prospect of the shale oil development technology iteration in continental rift lake basins is proposed.It is suggested to continuously strengthen the overall three-dimensional development,improve the precision level of engineering technology,upgrade the engineering technical indicator system,accelerate the intelligent optimization of engineering equipment,explore the application of complex structure wells,form a whole-process integrated quality management system from design to implementation,and constantly innovate the concept and technology of shale oil development,so as to promote the realization of extensive,beneficial and high-quality development of shale oil in continental rift lake basins.

A case study of continental shelf waves in the northwestern South China Sea induced by winter storms in 2021

This study aims to investigate characteristics of continental shelf wave(CSW)on the northwestern continental shelf of the South China Sea(SCS)induced by winter storms in 2021.Mooring and cruise observations,tidal gauge data at stations Hong Kong,Zhapo and Qinglan and sea surface wind data from January 1 to February 28,2021 are used to examine the relationship between along-shelf wind and sea level fluctuation.Two events of CSWs driven by the along-shelf sea surface wind are detected from wavelet spectra of tidal gauge data.The signals are triply peaked at periods of 56 h,94 h and 180 h,propagating along the coast with phase speed ranging from 6.9 m/s to18.9 m/s.The dispersion relation shows their property of the Kelvin mode of CSW.We develop a simple method to estimate amplitude of sea surface fluctuation by along-shelf wind.The results are comparable with the observation data,suggesting it is effective.The mode 2 CSWs fits very well with the mooring current velocity data.The results from rare current help to understand wave-current interaction in the northwestern SCS.

“Component flow”conditions and its effects on enhancing production of continental medium-to-high maturity shale oil

Based on the production curves,changes in hydrocarbon composition and quantities over time,and production systems from key trial production wells in lacustrine shale oil areas in China,fine fraction cutting experiments and molecular dynamics numerical simulations were conducted to investigate the effects of changes in shale oil composition on macroscopic fluidity.The concept of“component flow”for shale oil was proposed,and the formation mechanism and conditions of component flow were discussed.The research reveals findings in four aspects.First,a miscible state of light,medium and heavy hydrocarbons form within micropores/nanopores of underground shale according to similarity and intermiscibility principles,which make components with poor fluidity suspended as molecular aggregates in light and medium hydrocarbon solvents,such as heavy hydrocarbons,thereby decreasing shale oil viscosity and enhancing fluidity and outflows.Second,small-molecule aromatic hydrocarbons act as carriers for component flow,and the higher the content of gaseous and light hydrocarbons,the more conducive it is to inhibit the formation of larger aggregates of heavy components such as resin and asphalt,thus increasing their plastic deformation ability and bringing about better component flow efficiency.Third,higher formation temperatures reduce the viscosity of heavy hydrocarbon components,such as wax,thereby improving their fluidity.Fourth,preservation conditions,formation energy,and production system play important roles in controlling the content of light hydrocarbon components,outflow rate,and forming stable“component flow”,which are crucial factors for the optimal compatibility and maximum flow rate of multi-component hydrocarbons in shale oil.The component flow of underground shale oil is significant for improving single-well production and the cumulative ultimate recovery of shale oil.

Three-dimensional constrained gravity inversion of Moho depth and crustal structural characteristics at Mozambique continental margin

Mozambique's continental margin in East Africa was formed during the break-off stage of the east and west Gondwana lands. Studying the geological structure and division of continent-ocean boundary(COB) in Mozambique's continental margin is considered of great significance to rebuild Gondwana land and understand its movement mode. Along these lines, in this work, the initial Moho was fit using the known Moho depth from reflection seismic profiles, and a 3D multi-point constrained gravity inversion was carried out. Thus, highaccuracy Moho depth and crustal thickness in the study area were acquired. According to the crustal structure distribution based on the inversion results, the continental crust at the narrowest position of the Mozambique Channel was detected. According to the analysis of the crustal thickness, the Mozambique ridge is generally oceanic crust and the COB of the whole Mozambique continental margin is divided.

The formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3^(rd)submember of Chang 7 Member,Yanchang formation,Ordos Basin,China

Through microscopic analyses(e.g.,organic macerals,thin section observation,scanning electron microscope(SEM)imaging of fresh bedding planes via argon ion milling,and energy spectrum tests)combined with Rock-Eval analyses,this study systematically investigated the organic matter and pyrites in the continental shales in the 3^(rd)submember of the Chang 7 Member(Chang 7^(3)submember)in the Yanchang Formation,Ordos Basin and determined their types and the formation and evolutionary characteristics.The results are as follows.The organic matter of the continental shales in the Chang 7^(3)submember is dominated by amorphous bituminites and migrabitumens,which have come into being since the early diagenetic stage and middle diagenetic stage A,respectively.The formation and transformation of organic matter is a prerequisite for the formation of pyrites.The Ordos Basin was a continental freshwater lacustrine basin and lacked sulphates in waters during the deposition of the Chang 7 Member.Therefore,the syndiagenetic stage did not witness the formation of large quantities of pyrites.Since the basin entered early diagenetic stage A,large quantities of sulfur ions were released as the primary organic matter got converted into bituminites and,accordingly,pyrites started to form.However,this stage featured poorer fluid and spatial conditions compared with the syndepositional stage due to withdraw of water,the partial formation of bituminites,and a certain degree of compaction.As a result,large quantities of pyrrhotite failed to transition into typical spherical framboidal pyrites but grew into euhedral monocrystal aggregates.In addition,pyrites are still visible in the migrabitumens in both microfractures and inorganic pores of mudstones and shales,indicating that the pyrite formation period can extend until the middle diagenetic stage A.

Influence of Continental Atmospheric Forcing on the Decadal Variability of the West African Monsoon

The West African Monsoon (WAM) is characterized by strong decadal and multi-decadal variability and the impacts can be catastrophic for the local populations. One of the factors put forward to explain this variability involves the role of atmospheric dynamics, linked in particular to the Saharan Heat Low (SHL). This article addresses this question by comparing the sets of preindustrial control and historical simulation data from climate models carried out in the framework of the CMIP5 project and observations data over the 20th century. Through multivariate statistical analyses, it was established that decadal modes of ocean variability and decadal variability of Saharan atmospheric dynamics significantly influence decadal variability of monsoon precipitation. These results also suggest the existence of external anthropogenic forcing, which is superimposed on the decadal natural variability inducing an intensification of the signal in the historical simulations compared to preindustrial control simulations. We have also shown that decadal rainfall variability in the Sahel, once the influence of oceanic modes has been eliminated, appears to be driven mainly by the activity of the Arabian Heat Low (AHL) in the central Sahel, and by the structure of the meridional temperature gradient over the inter-tropical Atlantic in the western Sahel.

The Geoscience Frontier of Gulong Shale Oil:Revealing the Role of Continental Shale from Oil Generation to Production

The clay mineral content of Daqing Gulong shale is in the range of about 35%–45%,with particle sizes less than 0.0039 mm.The horizontal fluidity of oil in Gulong shale is poor,with near-zero vertical flowability.As a result,Gulong shale has been considered to lack commercial value.In recent years,however,interdisciplinary research in geoscience,percolation mechanics,thermodynamics,and surface mechanics has demonstrated that Gulong shale oil has a high degree of maturity and a high residual hydrocarbon content.The expulsion efficiency of Gulong shale in the high mature stage is 32%–48%.Favorable storage spaces in Gulong shale include connecting pores and lamellar fractures developed between and within organic matter and clay mineral complexes.The shale oil mainly occurs in micro-and nano-pores,bedding fractures,and lamellar fractures,with a high gas–oil ratio and medium–high movable oil saturation.Gulong shale has the characteristics of high hardness,a high elastic modulus,and high fracture toughness.This study achieves breakthroughs in the exploration and development of Gulong shale,including the theories of hydrocarbon generation and accumulation,the technologies of mobility and fracturing,and recoverability.It confirms the major transition of Gulong shale from oil generation to oil production,which has extremely significant scientific value and application potential for China’s petroleum industry.

Petrogenesis of the Late Eocene to Early Oligocene Yao'an Shoshonitic Complex,Southeastern Tibet:Partial Melting of an Ancient Continental Lithospheric Mantle beneath the Yangtze Block

Cenozoic potassic-ultrapotassic igneous rocks are widespread in the southeastern Tibetan Plateau.Their petrogenesis and magmatic processes remain subject to debate in spite of numerous publications.Almost all of the Cenozoic extrusive and intrusive rocks in the Yao’an area,western Yunnan Province,SW China,are geochemically shoshonitic,collectively termed here the Yao’an Shoshonitic Complex(YSC).The YSC is located in the(south)easternmost part of the ENE-WSW-trending,~550 km-long and~250 km-wide Cenozoic magmatic zone;the latter separates the orthogonal and oblique collision belts of the India-Eurasia collision orogen.Previously published geochronological and thermochronological data revealed that the rocks of the YSC were emplaced over a short timespan of 34-32 Ma.This and our new data suggest that the primary magma of the YSC likely was formed by partial melting of ancient continental lithospheric mantle beneath the Yangtze Block.This part of the continental lithospheric mantle had likely not been modified by any oceanic subduction.Fractionation crystallization of an Mg-and Ca-bearing mineral and TiFe oxides during the magmatic evolution probably account for the variable lithologies of the YSC.

Subaqueous volcanic eruptive facies,facies model and its reservoir significance in a continental lacustrine basin:A case from the Cretaceous in Chaganhua area of southern Songliao Basin,NE China

The conventional lithofacies and facies model of subaerial and marine pyroclastic rocks cannot reflect the characteristics of subaqueous volcanic edifice in lacustrine basins.In order to solve this problem,the lithofacies of subaqueous eruptive pyroclastic rocks is discussed and the facies model is established by taking the tuff cone of Cretaceous Huoshiling Formation in the Chaganhua area of the Changling fault depression,Songliao Basin as the research object.The results indicate that the subaqueous eruptive pyroclastic rocks in the Songliao Basin can be divided into two facies and four subfacies.The two facies are the subaqueous explosive facies and the volcanic sedimentary facies that is formed during the eruption interval.The subaqueous explosive facies can be further divided into three subfacies:gas-supported hot pyroclastic flow subfacies,water-laid density current subfacies and subaqueous fallout subfacies.The volcanic sedimentary facies consists of pyroclastic sedimentary rocks containing terrigenous clast subfacies.A typical facies model of the tuff cone that is formed by subaqueous eruptions in the Songliao Basin was established.The tuff cone is generally composed of multiple subaqueous eruption depositional units and can be divided into two facies associations:near-source facies association and far-source facies association.The complete vertical succession of one depositional unit of the near-source facies association is composed of pyroclastic sedimentary rocks containing terrigenous clast subfacies,gas-supported hot pyroclastic flow subfacies,water-laid density current subfacies and subaqueous fallout subfacies from bottom to top.The depositional unit of the far-source facies association is dominated by the subaqueous fallout subfacies and contains several thin interlayered deposits of the water-laid density current subfacies.The gas-supported hot pyroclastic flow subfacies and the pyroclastic sedimentary rocks containing terrigenous clast subfacies are favorable subaqueous eruptive facies for reservoirs in continental lacustrine basins.

Migration and accumulation mechanisms and main controlling factors of tight oil enrichment in a continental lake basin

Based on the typical dissection of various onshore tight oil fields in China,the tight oil migration and accumulation mechanism and enrichment-controlling factors in continental lake basins are analyzed through nuclear magnetic resonance(NMR)displacement physical simulation and Lattice Boltzmann numerical simulation by using the samples of source rock,reservoir rock and crude oil.In continental lake basins,the dynamic forces driving hydrocarbon generation and expulsion of high-quality source rocks are the foundational power that determines the charging efficiency and accumulation effect of tight oil,the oil migration resistance is a key element that influences the charging efficiency and accumulation effect of tight oil,and the coupling of charging force with pore-throat resistance in tight reservoir controls the tight oil accumulation and sweet spot enrichment.The degree of tight oil enrichment in continental lake basins is controlled by four factors:source rock,reservoir pore-throat size,anisotropy of reservoir structure,and fractures.The high-quality source rocks control the near-source distribution of tight oil,reservoir physical properties and pore-throat size are positively correlated with the degree of tight oil enrichment,the anisotropy of reservoir structure reveals that the parallel migration rate is the highest,and intralayer fractures can improve the migration and accumulation efficiency and the oil saturation.

Behavior and controlling factors of methane adsorption in Jurassic continental shale,northeastern Sichuan Basin

The behavior and controlling factors of natural gas adsorption in the Jurassic continental shale in the northeastern Sichuan Basin are studied based on the organic geochemical features,mineral compositions and pore structure parameters through a series of experiments on samples from the shale.Results show that the total gas content of the shale measured on-site is 0.1-5.3 cm^(3)/g,with an average of 0.7 cm^(3)/g.The methane isothermal adsorption curves show a trend of increasing first and then decreasing,indicating an obvious excessive adsorption.The shale has a maximum adsorption capacity(V^(L))of 0.44-3.59 cm^(3)/g,with an average of 1.64 cm^(3)/g,lower than that of marine shale in the same basin.The organic matter content and pore structure characteristics are identified as the two main factors controlling the adsorption capacity of the shale.Micropores in the shale are the main storage space for gas to be adsorbed.Due to well developed shell laminae and interlayers in the shale,calcite plays a more important role than clay minerals in affecting the adsorption of gas to the rock.The formation temperature and water content also significantly inhibit the gas adsorption to the shale.Compared with marine shale in the basin,the Jurassic continental shale is more heterogeneous and lower in TOC values.Furthermore,with a more widely developed clayey shale lithofacies and shell limy shale lithofacies as well as relatively less developed organic pores and micropores,the continental shale is inferior to marine shale in terms of gas adsorption capacity.

The gravity field and gravity data reduction across the continental area of Nigeria

This research presents the variation of the gravity field and associated gravity field components over the continental area of Nigeria to provide data for geoscience research,geodetic and engineering works,aerodynamic studies and deep crustal inferences.Accurate positions and elevations were observed at 58 of the 59 base stations of the Primary Gravity Network of Nigeria(PGNN),whose absolute gravity values had been accurately determined.The absolute gravity values were plotted against their respective positions to reveal the distribution pattern and strength of the gravity field within the study area.Theoretical gravity values at each base station were generated using the Somigliana's equation.The free-air gravity and free-air anomaly gravity values were generated with respect to the World Geodetic System 1984(WGS84)ellipsoid using GPS-derived elevation data.Then,the perturbing potential,free-air gravity with respect to the geoid,and the indirect effects were evaluated.The average of the indirect effects was used to adjust the WGS84 gravity formula to produce a gravity formula that better approximates the geoid across the continental area of Nigeria,compatible with the heights measured relative to the geoid,which can serve as a reference for establishing a vertical height control.The Bouguer gravity and Bouguer gravity anomalies across Nigeria revealed a“trans-southern gravity high strip”interpreted to be associated with mantle upwelling.Two new major mega-lineaments related to mantle upwelling were mapped.A batholith province trending NWeSE was delineated,occurring from north central Nigeria to the north western region and containing closures of“Bouguer gravity lows”interpreted as batholiths.A separate closure of“Bouguer gravity low”was detected at Azare,north eastern Nigeria,which may be due to the presence of intrusive granitic body.It is recommended that the mantle structure beneath“the trans-southern gravity high strip”,“delineated batholith province”and“isolated gravity closures”around the northeast of Nigeria should be studied from seismic shear wave splitting analysis for better understanding of the deep lithospheric structures and moho relief.

Hydrogeological Characterization of Continental Terminal and Oligo-Miocene Aquifers in the Tambacounda-Kaffrine Zone(Senegal)

This study aims to make a hydrogeological characterization of the aquifers of the Continental Terminal and the Oligo-Miocene.To do so,an analysis is conducted on the basis of hydrogeological parameters from 172 boreholes,10 of which are used for groundwater levels and flows analysis.The results of the statistical analysis of the hydrogeological parameters show that the average flow rate is 42.29 m^(3)/h,the average specific flow rate is 5.96 m^(3)/h/m,and the average transmissivity is 0.024 m^(2)/s.These values highlight the high productivity of aquifers from the Continental Terminal and the Oligo-Miocene.The results of piezometry showed that water flows from the south center to the northwest of Tambacounda where the largest depression is located and could even be the outlet of the system.The groundwater fluctuations between low water level and high water level seasons reveal a rise in the piezometric surface of the aquifers at the scale of the study area.

Study of the Akouedo Landfill (Abidjan) Using the Electrical Resistivity Method: Implications for the Risk of Contamination of the Continental Terminal Water Table

An electrical resistivity survey was carried out on the household and industrial waste disposal site (landfill) of Akouédo (Central Abidjan) with a view to searching for a possible layer of clay in the stratification which could constitute a protective screen of the aquifer of the Continental Terminal of Abidjan against the migration of leachate. Electrical surveys (SE) carried out according to the Schlumberger configuration showed that the stratigraphy of the area is composed of three to four geoelectric layers depending on the SE positions. The correlation with the lithology of two piezometric boreholes carried out indicates that the lithology of the study area is dominated by clayey sand, sand, sandy clay and clay. The average thickness of accumulated waste varies between 30 and 40 m. The virtual absence of a continuous layer of clay under the waste exposes the Continental Terminal aquifer to contamination by leachate from waste accumulated over several decades in the Akouedo area.

A Review on the Study of Continental Drift and Numerical Simulation Associated with the Early Earth Core-Magma Angular Momentum Exchange

According to the drive of planetary-scale upper magma fluid motions associated with the core-magma angular momentum exchange in the early Earth’s interior, this paper reviewed the results of continental drift studied over the last three decades. The theoretical speculation is in good fit to the traces of geological events left on the Earth’s surface. A northeastward drift directionality of the Australian, African, and South American continents relative to the Antarctica Continent in the Southern Hemisphere is reanalyzed according to the slowing down of the early Earth’s rotation. Six traces of significant back-and-forth drifts of the Australian and Asian continents left respectively on the Southwest and Northwest Pacific seafloors are reidentified according to the gradually decreasing amplitude of core-magma angular momentum exchange during early geological evolution. Finally, the thickening and shortening of different continents during the early drift processes are re-simulated by using a simple magma fluid dynamical model.

Metallogeny and Emplacement Conditions of Continental Terminal 3 (Ct3) Iron Formations of the Niamey Region (Western Niger)

This study aims to characterize the different lithofacies of the Ct3 formation in the Niamey region, and to determine the distribution of major and trace elements, in order to highlight the conditions for the establishment of iron mineralization. A lithological column, synthesizing sections of selected outcrops in the vicinity of Niamey, was produced. The chemical compositions of the selected samples were determined by X-ray fluorescence (XRF) spectrometry. Microscopic analysis of the thin sections determined the gœthitic nature of the oolitic iron ore. The oolites show a quartz, limonitic or gœthitic nucleus. Sometimes the nucleus is absent. From a morphoscopic point of view, two types of oolites have been distinguished: spherical-shaped and ellipsoidal-shaped oolites. The oolites are either contiguous or disseminated, as the case may be, in a limonitic to goethitic cement or in a fine sandstone matrix. The larger oolites (pisolites) are relatively friable. They reflect the influence of a relatively turbulent to submerged environment. The hardground of the iron mineralized horizons are covered by quartz grains. They are indicative of a submerged or emergent environment. X-ray fluorescence analysis shows high Fe2O3 contents (50% to 80%) and variable contents of major elements SiO2, Al2O3, TiO2, MnO, MgO, CaO, K2O and P2O5 associated with certain trace elements such as Th, U, V, Y, Zn, Zr and As. The results of the study are an important tool for decision-makers to adopt effective prevention/remediation measures for groundwater contamination in the Continental terminal aquifer systems.

The 3D magnetic structure beneath the continental margin of the northeastern South China Sea

Understanding the continental margin of the Northeastern South China Sea is critical to the study of deep structures, tectonic evolution, and dynamics of the region. One set of important data for this endeavor is the total-field magnetic data. Given the challenges associated with the magnetic data at low latitudes and with remanent magnetism in this area, we combine the equivalent-source technique and magnetic amplitude inversion to recover 3D subsurface magnetic structures. The inversion results show that this area is characterized by a north-south block division and east-west zonation. Magnetic regions strike in EW, NE and NW direction and are consistent with major tectonic trends in the region. The highly magnetic zone recovered from inversion in the continental margin differs visibly from that of the magnetically quiet zones to the south. The magnetic anomaly zone strikes in NE direction, covering an area of about 500 km × 60 km, and extending downward to a depth of 25 km or more. In combination with other geophysical data, we suggest that this strongly magnetic zone was produced by deep underplating of magma associated with plate subduction in Mesozoic period. The magnetically quiet zone in the south is an EW trending unit underlain by broad and gentle magnetic layers of lower crust. Its magnetic structure bears a clear resemblance to oceanic crust, assumed to be related to the presence of ancient oceanic crust there.

Late Neoarchean crustal growth under paired continental arc-back arc system in the North China Craton

The late Archean(~3.0-2.5 Ga)was a key period of continental growth globally,which is widely considered to reflect the onset of vigorous plate tectonic activity,although related continental growth modes remain contentious.Here we investigate a suite of late Neoarchean metavolcanic rocks from the southwest Qixia area of the Jiaobei terrane in the North China Craton.The rocks in this suite include amphibolites,clinopyroxene amphibolites,and hornblende plagioclase gneisses.We present zircon U-Pb isotopic data which indicate that the protoliths of these rocks formed during~2549-2511 Ma.The(clinopyroxene)amphibolites correspond to meta-basaltic rocks,with some containing high modal content of titanite.These rocks show moderate to high FeO_(T)(8.96-13.62 wt.%)and TiO_(2)(0.59-1.59 wt.%),flat to less fractionated REE patterns,and mildly negative Th,Nb,and Ta anomalies,resembling those of Fe-tholeiites.In addition,they display positive zirconε_(Hf)(t)values(+2.6 to+8.7),and are devoid of crustal contamination or fractional crystallization.Combined with the low Nb/Yb(mostly<1.60)and(Hf/Sm)_N(mostly<0.95),low to moderate Th/Yb(0.08-0.54),and low V/Sc(5.53-9.19)ratios,these basaltic rocks are interpreted to have been derived from a relatively reduced and depleted mantle source that was mildly metasomatized by hydrous fluids.The hornblende plagioclase gneisses are meta-andesitic rocks,and occur interlayered with the basaltic rocks.They are transitional between tholeiitic and calc-alkaline rock series,and show fractionated REE patterns with evidently negative Th,Nb,and Ta anomalies.The depleted zirconε_(Hf)(t)values(+2.4 to+8.4)and quantitative chemical modeling suggest that the andesitic rocks were most likely generated by injection and mixing of juvenile felsic magmas with the tholeiitic basaltic magmas.In general,the chemical features and genesis of late Neoarchean meta-basaltic rocks in our study area resemble those of Mariana back-arc basin basalts.Combined with regional geological data,it is proposed that the Jiaobei terrane witnessed late Neoarchean crustal growth under a paired continental arc-back arc setting.On a regional context,we propose two distinct geodynamic mode of late Neoarchean continental growth across North China Craton(particularly the Eastern Block),i.e.,(1)arc-continent accretion along northwestern part of the Eastern Block;and(2)paired continental arc-back arc system surrounding the~3.8-2.7 Ga continental nuclei to the southeast.

In situ characterization of forearc serpentinized peridotite from the Sulu ultrahigh-pressure terrane: Behavior of fluid-mobile elements in continental subduction zone

Serpentinized peridotites in the Yangkou(YK),Suoluoshu(SLS) and Hujialin(HJL) areas in the Sulu ultrahighpressure terrane represent the relic of ancient subcontinental lithospheric mantle below the North China Craton.Their protoliths,harzburgite and dunite,were variably hydrated by aqueous fluids released from subducting Yangtze continent.The rocks are enriched in fluid-mobile elements(FME) including Sb(42–333 times the depleted mantle value) and Pb(30–476 times).The degrees of the FME enrichment are comparable to that of the Himalayan forearc serpentinites,and greater than forearc mantle serpentinites from Marianas,suggesting that the degrees of FME enrichment in the forearc serpentinites are greater in continental subduction zones than those in the oceanic subduction zones.Lizardite after olivine in the SLS serpentinite shows higher degrees of enrichment in Sb and As than those for antigorite after both olivine and orthopyroxene in the YK area.The antigorite has highly enriched in Pb,U,Cs,and LREE,but not for the lizardite.The abundance of FME in two different species of serpentine reflects the different temperature of hydration.At temperature lower than 300 ℃,formed lizardite at shallow depths of the mantle wedge incorporates elements that are fluid mobile at low temperatures,such as Sb and As.When the temperature greater than 300 ℃,formed antigorite at a relatively deep mantle wedge incorporate more FME from the subducting continental slab(or fragments),including Pb,U,Cs,LREE as well as Sb and As.The eventual breakdown of antigorite(600–700 ℃) in prograde metamorphism would discharge water as well as FME into the subducting channel and/or the overlying mantle.